Ceramic heater

ABSTRACT

Aluminum nitride, silicon nitride or silicon carbide is employed as the main component forming a substrate for increasing mechanical strength and improving thermal shock resistance, a proper additive is blended for controlling thermal conductivity and a temperature gradient from a heating element to an electrode is loosened for providing a dimensional ratio of the substrate effective for preventing oxidation of a contact between an electrode of the heating element and a connector of a feeding part. In a ceramic heater having an electrode and a heating element formed on the surface of a ceramic substrate, A/B≧20 is satisfied assuming that A represents the distance from a contact between a circuit of the heating element and the electrode to an end of the ceramic substrate closer to the electrode and B represents the thickness of the ceramic substrate, and the thermal conductivity of the ceramic substrate is adjusted to 30 to 80 W/m·K.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a ceramic heater having a heating element formed on a ceramic substrate (hereinafter simply referred to as a substrate), and more particularly, it relates to a ceramic heater usefully applied to an electric or electronic apparatus.

[0003] 2. Description of the Prior Art

[0004] In general, ceramics having an excellent insulation property and a high degree of freedom in design of a heater circuit is applied to various types of heater substrates. In particular, an alumina sintered body, having high mechanical strength among ceramic materials with thermal conductivity reaching 30 W/m·K, relatively excellent in thermal conductivity and thermal shock resistance and obtained at a low cost, is widely employed. When the alumina sintered body is applied to a substrate, however, the substrate cannot follow abrupt temperature change of a heating element and may be broken due to a thermal shock.

[0005] Japanese Patent Laying-Open No. 4-324276 (1992) discloses a ceramic heater employing aluminum nitride having thermal conductivity of at least 160 W/m·K. A substrate having such a degree of thermal conductivity is not broken by abrupt temperature change dissimilarly to the substrate of alumina. This gazette describes that the uniform heating property of the overall heater can be secured by stacking about four layers of aluminum nitride and forming heating elements having different shapes on the respective layers while locating an electrode substantially at the center of the substrate for uniformizing temperature distribution in the ceramic heater.

[0006] Japanese Patent Laying-Open No. 9-197861 (1997) discloses employment of aluminum nitride for a substrate of a heater for a fixing device. According to this prior art, a substrate having thermal conductivity of at least 50 W/m·K, preferably at least 200 W/m·K can be obtained by setting the mean particle diameter of aluminum nitride particles to not more than 6.0 μm, optimizing combination of sintering agents and performing sintering at a temperature of not more than 1800° C., preferably not more than 1700° C. This gazette describes that the substrate having excellent thermal conductivity is employed for the heater for a fixing device thereby efficiently transferring heat of a heating element to paper or toner and improving a fixing rate.

[0007] In addition, Japanese Patent Laying-Open No. 11-95583 (1999) discloses employment of silicon nitride for a substrate of a heater for a fixing device. This prior art reduces the thickness of the substrate itself by employing silicon nitride having relatively high strength with flexural strength of 490 to 980 N/mm² and thermal conductivity of at least 40 W/m·K, preferably at least 80 W/m·K and reducing heat capacity thereby reducing power consumption. This gazette describes that silicon nitride has lower in thermal conductivity than aluminum nitride and hence heat of a heating element is not readily transmitted to a connector of a feeding part but an electrode of the heating element can be prevented from oxidation for avoiding a contact failure.

[0008] When thermal conductivity of a substrate is increased, the quantity of diffusion to parts other than a heating part is also increased although heat propagation efficiency from a heating element is improved, to consequently increase power consumption. In order to prevent oxidation of a contact between an electrode of the heating element and a connector of a feeding part, therefore, it is effective that a uniform heating property around the substrate is excellent and a temperature around the electrode of the heating element is lower by at least several % than that of the heating element region.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a ceramic heater increased in mechanical strength of a substrate and improved in thermal shock resistance.

[0010] Another object of the present invention is to provide a ceramic heater capable of controlling thermal conductivity of a substrate and loosening a temperature gradient from a heating element to an electrode thereby preventing oxidation of a contact between the electrode of the heating element and a connector of a feeding part.

[0011] In a ceramic heater according to the present invention, a ceramic substrate provided with an electrode and a heating element on its surface is formed in a shape satisfying A/B≧20 assuming that A represents the distance from a contact between the heating element and the electrode to an end of the substrate closer to the electrode and B represents the thickness of the substrate, and the thermal conductivity of the substrate is adjusted to 30 to 80 W/m·K.

[0012] The main component forming the substrate is aluminum nitride, silicon nitride or silicon carbide, and a subsidiary component having thermal conductivity of not more than 50 W/m·K is added thereto.

[0013] If the main component of the ceramic is aluminum nitride, 5 to 100 parts by weight of aluminum oxide, 1 to 20 parts by weight of silicon and/or a silicon compound in terms of silicon dioxide or 5 to 100 parts by weight of zirconium and/or a zirconium compound in terms of zirconium oxide is added to 100 parts by weight of aluminum nitride, in order to adjust thermal conductivity thereof.

[0014] In order to obtain a ceramic sintered body having high mechanical strength, 1 to 10 parts by weight of an alkaline earth element and/or a rare earth element of the periodic table is introduced as a sintering agent with respect to 100 parts by weight of aluminum nitride. Calcium (Ca) is preferably selected as the alkaline earth element of the periodic table, while neodymium (Nd) or ytterbium (Yb) are preferably selected as the rare earth element of the periodic table.

[0015] The material for the substrate of the ceramic heater according to the present invention is preferably mainly composed of aluminum nitride (AlN), silicon nitride (Si₃N₄) or silicon carbide (SiC). While a substrate having thermal conductivity exceeding 100 W/m·K can be obtained by sintering material powder of such ceramic with addition of not more than several % of a proper sintering agent, the thermal conductivity of the substrate can be reduced to 30 to 80 W/m·K by adding a subsidiary component having thermal conductivity of not more than 50 W/m·K to the material powder.

[0016] If the thermal conductivity of the substrate is less than 30 W/m·K, there is a high possibility that the substrate itself is unpreferably broken by a thermal shock due to abrupt temperature increase of the heating element as energized. If the thermal conductivity of the substrate exceeds 80 W/m·K, the heat of the heating element is propagated to the overall substrate to unpreferably increase the quantity of diffusion to parts other than a heating part while also increasing power consumption, although a uniform heating property is excellent.

[0017] When adding aluminum oxide (Al₂O₃) to aluminum nitride (AlN), it is preferably to add 5 to 100 parts by weight of the former with respect to 100 parts by weight of the latter. The added aluminum oxide solidly dissolves oxygen in aluminum nitride in the sintered body thereby reducing the thermal conductivity while aluminum oxide having thermal conductivity of about 20 W/m·K itself is present in a grain boundary phase of aluminum nitride to effectively reduce the thermal conductivity of the ceramic sintered body. If the content of aluminum oxide is less than 5 parts by weight, the thermal conductivity may exceed 80 W/m·K. If the content of aluminum oxide exceeds 100 parts by weight, aluminum nitride reacts with aluminum oxide to form aluminum oxynitride. This substance has extremely low thermal conductivity, and hence the thermal conductivity of the overall substrate may be less than 30 W/m·K in this case.

[0018] Silicon and/or a silicon compound can be added to aluminum nitride (AlN) for adjusting the thermal conductivity. Silicon dioxide (SiO₂), silicon nitride (Si₃N₄) or silicon carbide (SiC) may be employed as the added silicon compound. Such a substance is present in a grain boundary phase in the sintered body, and serves as a thermal barrier phase inhibiting thermal conduction between aluminum nitride particles. Such silicon and/or a silicon compound is preferably added by 1 to 20 parts by weight in terms of silicon dioxide (SiO₂) with respect to 100 parts by weight of aluminum nitride. If the content of silicon and/or a silicon compound is less than 1 part by weight, the thermal barrier effect of silicon tends to be insufficient and hence the thermal conductivity may exceed 80 W/m·K. If the content of silicon and/or a silicon compound exceeds 20 parts by weight, the thermal conductivity tends to be less than 30 W/m·K.

[0019] Zirconium and/or a zirconium compound can be added to aluminum nitride (AlN) for adjusting the thermal conductivity. A typical example is zirconium oxide (ZrO₂). This substance is present in a grain boundary phase in the sintered body and serves as a thermal barrier phase inhibiting thermal conduction between aluminum nitride particles. 5 to 100 parts by weight of zirconium oxide is preferably added with respect to 100 parts by weight of aluminum nitride. If the content of zirconium oxide is less than 5 parts by weight, the thermal barrier effect of zirconium tends to be insufficient and hence the thermal conductivity may exceed 80 W/m·K. If the content of zirconium exceeds 100 parts by weight, the thermal conductivity tends to be less than 30 W/m·K.

[0020] Titanium oxide, vanadium oxide, manganese oxide or magnesium oxide can also be added as another subsidiary component, in order to reduce the thermal conductivity of aluminum nitride. 15 to 30 parts by weight of titanium oxide, 5 to 20 parts by weight of vanadium oxide, 5 to 10 parts by weight of manganese oxide or 5 to 15 parts by weight of magnesium oxide is preferably added with respect to 100 parts by weight of aluminum nitride.

[0021] Also when the ceramic is mainly composed of silicon nitride (Si₃N₄), aluminum oxide, zirconium oxide, titanium oxide, vanadium oxide, manganese oxide or magnesium oxide can be added for adjusting thermal conductivity. 2 to 20 parts by weight of aluminum oxide, 5 to 20 parts by weight of zirconium oxide, 10 to 30 parts by weight of titanium oxide, 5 to 20 parts by weight of vanadium oxide, 5 to 10 parts by weight of manganese oxide or 10 to 20 parts of magnesium oxide is preferably added with respect to 100 parts by weight of silicon nitride.

[0022] When the ceramic is mainly composed of silicon carbide (SiC), aluminum oxide, zirconium oxide, titanium oxide, vanadium oxide, manganese oxide or magnesium oxide can be added for adjusting thermal conductivity. 10 to 40 parts by weight of aluminum oxide, 5 to 20 parts by weight of zirconium oxide, 15 to 30 parts by weight of titanium oxide, 10 to 25 parts by weight of vanadium oxide, 2 to 10 parts by weight of manganese oxide or 5 to 15 parts of magnesium oxide is preferably added with respect to 100 parts by weight of silicon carbide.

[0023] When the main component is prepared from aluminum nitride (AlN) in the present invention, at least 1 part by weight of an alkaline earth element and/or a rare earth element of the periodic table is preferably introduced as a sintering agent with respect to 100 parts by weight of material powder of the main component, in order to obtain a dense sintered body. The alkaline earth element of the periodic table is preferably calcium (Ca), while the rare earth element of the periodic table is preferably neodymium (Nd) or ytterbium (Yb). Sintering can be performed at a relatively low temperature by adding such element(s), for reducing the sintering cost.

[0024] According to the present invention, the sintering body may be prepared by a well-known method. For example, an organic solvent, a binder etc. may be added to a prescribed quantity of material powder for preparing a slurry through a mixing step in a ball mill, forming the slurry into a sheet of a prescribed thickness by the doctor blade method, cutting the sheet into a prescribed size/shape, degreasing the cut sheet in the atmosphere or in nitrogen, and thereafter sintering the sheet in a non-oxidizing atmosphere.

[0025] The slurry can be formed through general means such as pressing or extrusion molding. In order to prepare the heater, the heating element can be formed in a prescribed pattern by sintering a layer of a high melting point metal consisting of tungsten or molybdenum on the sintered body by a technique such as screen printing in a non-oxidizing atmosphere. The electrode serving as a feeding part for the heating element can also be simultaneously formed by screen-printing the same on the sintered body. In this case, however, degreasing must be performed in a non-oxidizing atmosphere of nitrogen or the like in order to prevent oxidation of a metallized layer. Further, Ag or Ag—Pd can be employed as the heating element. While Examples of the present invention are described with reference to ceramic heaters for soldering irons, the present invention is not restricted to this application.

[0026] In the ceramic heater according to the present invention, the thermal conductivity of the substrate is adjusted to 30 to 80 W/m·K and the relation between the distance A from the contact of the circuit of the heating element on the substrate to the end of the substrate closer to the electrode and the thickness B of the substrate is set to satisfy A/B≧20, thereby increasing mechanical strength of the substrate, improving thermal shock resistance, loosening a temperature gradient from the heating element to the electrode, inhibiting oxidation of the contact of the electrode part and preventing a contact failure.

[0027] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a plan view of a ceramic heater according to the present invention;

[0029]FIG. 2 is a sectional view of the ceramic heater taken along the line II-II in FIG. 1; and

[0030]FIG. 3 is a sectional view of a heater for a soldering iron according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

[0031] In each sample, the quantity of aluminum oxide (Al₂O₃) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic was selected as shown in Table 1, while 2 parts by weight of Yb₂O₃, 2 parts by weight of Nd₂O₃ and 0.3 parts by weight of CaO were added as sintering agents with addition of an organic solvent and a binder, and these materials were mixed in a ball mill for 24 hours. A slurry obtained in this manner was formed into a sheet by the doctor blade method so that the thickness after sintering was 0.7 mm.

[0032] The sheet was cut so that the dimensions of both substrates 1 a and 1 b shown in a plan view of a ceramic heater in FIG. 1 were 50 mm by 5 mm after sintering, and degreased in the atmosphere at 500° C. Then, the degreased body was sintered in a nitrogen atmosphere at 1800° C., and thereafter polished into a thickness (B) of 0.5 mm. Further, a heating element 2 and an electrode 3 were screen-printed on the substrate 1 a with Ag—Pd paste and Ag paste respectively, and sintered in the atmosphere at 880° C. As to the size/shape of the ceramic heater, the longitudinal length of the circuit of the heating element 2 was set to 40 mm for satisfying the condition A/B≧20 assuming that A represents the distance from the contact between the heating element 2 and the electrode 3 to an end of the substrate 1 a closer to the electrode 3 and B represents the thickness of the substrate 1 a.

[0033] Further, pasty sealing glass 4 was applied in order to protect the heating element 2 as shown in FIG. 2, the substrate 1 b of 45 mm by 5 mm was placed thereon and sintered in the atmosphere at 880° C. for bonding the substrates 1 a and 1 b to each other, thereby preparing a heater for a soldering iron 10 shown in a sectional view of FIG. 3. The substrates 1 a and 1 b, made of ceramic, are identical in size and material to each other except slight difference between the total lengths thereof. Table 1 shows values of thermal conductivity in Example 1 measured by applying a laser flash method to the substrate 1 a.

[0034] On the forward end of the soldering iron 10, a frame 12 of a metal thin plate holds a tip 11 consisting of the substrates 1 a and 1 b. A heat insulator 13 consisting of mica or asbestos is interposed between the frame 12 and the tip 11, while a wooden handle 14 is engaged with the outer periphery of the frame 12. In order to connect the electrode 3 with a lead wire 15, a contact 16 on the side of the lead wire 15 is brought into pressure contact with the electrode 3 by a spring seat 17 and a clamp bolt 18 for attaining mechanical contact bonding since a deposited metal such as solder is readily thermally deteriorated. If the temperature is repeatedly increased beyond 300° C. in the atmosphere, the contact 16 is oxidized to readily cause a contact failure. Numeral 19 denotes a window for observing the temperature of the part of the electrode 3.

[0035] While the material for the tip 11 of the soldering iron 10 is generally prepared from copper due to excellent affinity with solder and high thermal conductivity, adhesion of solder is readily caused due to the excellent affinity with solder. When the tip 11 must not be covered with solder in a specific application, therefore, the material therefor is prepared from ceramic. The solder, which is prepared from an alloy of tin and lead while the melting point thereof is reduced as the content of tin is increased, is generally welded at a temperature of about 230 to 280° C. A toner fixing temperature of a heater for a fixing device is 200 to 250° C.

[0036] The quantity of current was adjusted with a sliding voltage regulator so that the temperature of a portion of the soldering iron 10 where the tip 11 was exposed was stabilized at 300° C., for measuring power consumption. At the same time, the current temperature of the part of the electrode 3 was measured with an infrared radiation thermometer through the window 19 for temperature observation. Table 1 also shows the results. TABLE 1 Content of Al₂O₃ Thermal Temperature of Power Sample (parts by Conductivity Electrode Part Consumption at No. weight) (W/m · K) (° C.) 300° C. (W) ⋆1 0 148 232 120 ⋆2 4 99 241 105 3 5 80 273 80 4 10 72 277 75 5 25 50 281 73 6 70 37 283 70 7 100 30 285 68 ⋆8 120 20 — substrate cracked upon energization

[0037] Referring to Table 1, power consumption increased in samples Nos. 1 and 2 having thermal conductivity exceeding the upper limit of the present invention, while a crack similar to a quenching crack frequently observed in earthenware was caused in the substrate 1 a of a sample No. 8 having thermal conductivity less than the lower limit due to a thermal shock. The temperature gradient of the part of the electrode 3 with respect to the heating element 2 was loose within the range of thermal conductivity recommended in the present invention, to indicate that the uniform heating property of the substrate 1 a is excellent.

EXAMPLE 2

[0038] In each sample, the quantities of silicon dioxide (SiO₂), silicon nitride (Si₃N₄) and silicon carbide (SiC) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic were selected as in Table 2, while 2 parts by weight of Yb₂O₃, 2 parts by weight of Nd₂O₃ and 0.3 parts by weight of CaO were added as sintering agents for preparing a substrate by a method similar to that in Example 1. The substrate was assembled into the soldering iron 10 shown in FIG. 3, and the characteristics of the substrate serving as a ceramic heater were evaluated through a procedure similar to that in Example 1. Table 2 also shows the results. TABLE 2 Content in Thermal Temperature Power Sample Terms of SiO₂ Conductivity of Electrode Consumption at No. Additive (parts by weight) (W/m · K) Part (° C.) 300° C. (W) ⋆9 SiO₂ 0.5 120 237 111 ⋆10 Si₃N₄ 0.5 131 235 115 ⋆11 SiC 0.5 118 238 108 12 SiO₂ 1.0 75 276 72 13 Si₃N₄ 1.0 79 275 75 14 SiC 1.0 74 277 72 15 SiO₂ 5.0 63 279 70 16 Si₃N₄ 10.0 58 280 68 17 SiO₂ 15.0 41 281 65 18 SiC 20.0 32 285 63 19 SiO₂ 20.0 33 284 63 ⋆20 SiO₂ 25.0 24 — substrate cracked upon energization ⋆21 Si₃N₄ 25.0 27 — substrate cracked upon energization

[0039] Referring to Table 2, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 12 to 19 having contents of additives in terms of SiO₂ within the range recommended in the present invention. The temperature gradient of the part of the electrode 3 with respect to the heating element 2 also exhibited a stable uniform heating property.

EXAMPLE 3

[0040] In each sample, the quantity of zirconium dioxide (ZrO₂) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic was selected as shown in Table 3, while 2 parts by weight of Yb₂O₃, 2 parts by weight of Nd₂O₃ and 0.3 parts by weight of CaO were added as sintering agents for preparing a substrate by a method similar to that in Example 1. Table 3 shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron 10 shown in FIG. 3 evaluated through a procedure similar to that in Example 1. TABLE 3 Content of ZrO₂ Thermal Temperature of Power Sample (parts by Conductivity Electrode Part Consumption at No. weight) (W/m · K) (° C.) 300° C. (W) ⋆22 4 104 238 113 23 5 77 275 78 24 10 70 278 72 25 25 65 280 71 26 70 45 282 69 27 100 32 284 68 ⋆28 120 19 — substrate cracked upon energization

[0041] Referring to Table 3, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 23 to 27 having contents of zirconium oxide (ZrO₂) within the range recommended in the present invention. The temperature gradient of the part of the electrode 3 with respect to the heating element 2 also exhibited a stable uniform heating property.

EXAMPLE 4

[0042] In each sample, the quantities of aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), titanium dioxide (TiO₂), vanadium oxide (V₂O₅), manganese dioxide (MnO₂) and magnesium oxide (MgO) added to 100 parts by weight of silicon nitride (Si₃N₄) forming the main component of ceramic were selected as shown in Table 4, while 10 parts by weight of yttrium oxide was added as a sintering agent for forming a sheet by a method similar to that in Example 1. Thereafter the sheet was degreased in a nitrogen atmosphere at 850° C., and sintered in a nitrogen atmosphere of 1850° C. for three hours thereby preparing each substrate shown in Table 4. Table 4 also shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron 10 shown in FIG. 3 evaluated through a procedure similar to that in Example 1. TABLE 4 Thermal Temperature Power Sample Content Conductivity of Electrode Consumption at No. Additive (parts by weight) (W/m · K) Part (° C.) 300° C. (W) ⋆29 — — 100 239 111 30 Al₂O₃ 2 79 273 80 31 Al₂O₃ 5 52 280 73 32 Al₂O₃ 10.0 41 283 71 33 Al₂O₃ 20.0 31 284 69 ⋆34 Al₂O₃ 30.0 15 — substrate cracked upon energization 35 ZrO₂ 5.0 75 274 80 36 ZrO₂ 10.0 51 281 74 37 ZrO₂ 20.0 35 284 72 ⋆38 ZrO₂ 30.0 19 — substrate cracked upon energization 39 TiO₂ 10.0 74 275 78 40 TiO₂ 30.0 45 282 72 ⋆41 TiO₂ 50.0 26 — substrate cracked upon energization 42 V₂O₅ 10.0 72 275 80 43 V₂O₅ 20.0 43 285 72 ⋆44 V₂O₅ 30.0 unsinterable — — 45 MnO₂ 5.0 69 277 77 46 MnO₂ 10.0 35 285 71 ⋆47 MnO₂ 20.0 23 — substrate cracked upon energization 48 MgO 10.0 74 274 80 49 MgO 20.0 53 279 75 ⋆50 MgO 30.0 23 — substrate cracked upon energization

[0043] Referring to Table 4, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 30 to 33, 35 to 37, 39 and 40, 42 and 43, 45 and 46 and 48 and 49 having contents of the additives within the range recommended in the present invention. The temperature gradient of the part of the electrode 3 with respect to the heating element 2 also exhibited a stable uniform heating property.

EXAMPLE 5

[0044] In each sample, the quantities of aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), titanium dioxide (TiO₂), vanadium oxide (V₂O₅), manganese dioxide (MnO₂) and magnesium oxide (MgO) added to 100 parts by weight of silicon carbide (SiC) forming the main component of ceramic were selected as shown in Table 5, while 1.0 part by weight of boron carbide (B₄C) was added as a sintering agent for forming a sheet by a method similar to that in Example 1. Thereafter the sheet was degreased in a nitrogen atmosphere at 850° C., and sintered in an argon atmosphere of 2000° C. for three hours thereby preparing each substrate shown in Table 5. Table 5 also shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron 10 shown in FIG. 3 evaluated through a procedure similar to that in Example 1. TABLE 5 Thermal Temperature Power Sample Content Conductivity of Electrode Consumption at No. Additive (parts by weight) (W/m · K) Part (° C.) 300° C. (W) ⋆51 — — 162 221 132 52 Al₂O₃ 10.0 79 269 82 53 Al₂O₃ 20.0 61 275 77 54 Al₂O₃ 30.0 46 280 72 55 Al₂O₃ 40.0 32 285 69 ⋆56 Al₂O₃ 50.0 16 — substrate cracked upon energization 57 ZrO₂ 5.0 74 271 83 58 ZrO₂ 10.0 49 279 76 59 ZrO₂ 20.0 33 285 73 ⋆60 ZrO₂ 30.0 17 — substrate cracked upon energization 61 TiO₂ 15.0 78 269 82 62 TiO₂ 30.0 48 280 76 ⋆63 TiO₂ 50.0 26 — substrate cracked upon energization 64 V₂O₅ 10.0 69 272 79 65 V₂O₅ 25.0 39 283 71 ⋆66 V₂O₅ 40.0 18 — substrate cracked upon energization 67 MnO₂ 2.0 77 270 83 68 MnO₂ 10.0 42 282 71 ⋆69 MnO₂ 20.0 21 — substrate cracked upon energization 70 MgO 5.0 70 270 82 71 MgO 15.0 51 278 77 ⋆72 MgO 30.0 24 — substrate cracked upon energization

[0045] Referring to Table 5, the thermal conductivity was adjusted in the range and the power consumption was suppressed in samples Nos. 52 to 55, 57 to 59, 61 and 62, 64 and 65, 67 and 68 and 70 and 71 having contents of the additives within the range recommended in the present invention. The temperature gradient of the part of the electrode 3 with respect to the heating element 2 also exhibited a stable uniform heating property.

EXAMPLE 6

[0046] In each sample, the quantities of titanium dioxide (TiO₂), vanadium oxide (V₂O₅), manganese dioxide (MnO₂) and magnesium oxide (MgO) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic were selected as shown in Table 6, while 2 parts by weight of Yb₂O₃, 2 parts by weight of Nd₂O₃ and 0.3 parts by weight of CaO were added as sintering agents for preparing a substrate by a method similar to that in Example 1. Table 6 also shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron 10 shown in FIG. 3 evaluated through a procedure similar to that in Example 1. TABLE 6 Thermal Temperature Power Sample Content Conductivity of Electrode Consumption at No. Additive (parts by weight) (W/m · K) Part (° C.) 300° C. (W) ⋆73 TiO₂ 5.0 123 235 112 74 TiO₂ 15.0 74 275 77 75 TiO₂ 30.0 40 282 73 ⋆76 TiO₂ 50.0 23 — substrate cracked upon energization 77 V₂O₅ 5.0 70 278 74 78 V₂O₅ 20.0 36 283 70 ⋆79 V₂O₅ 40.0 17 271 substrate cracked upon energization 80 MnO₂ 5.0 71 277 74 81 MnO₂ 10.0 47 285 73 ⋆82 MnO₂ 20.0 22 — substrate cracked upon energization 83 MgO 5.0 67 279 73 84 MgO 15.0 49 281 72 ⋆85 MgO 30.0 18 — substrate cracked upon energization

[0047] Referring to Table 6, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 74 and 75, 77 and 78, 80 and 81 and 83 and 84 having contents of the additives within the range recommended in the present invention. The temperature gradient of the part of the electrode 3 with respect to the heating element 2 also exhibited a stable uniform heating property.

EXAMPLE 7

[0048] Substrates similar to that shown in FIG. 1 were formed by samples Nos. 2a, 2b and 2c prepared by adding 4 parts by weight of aluminum oxide (Al₂O₃) to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic, samples Nos. 5a, 5b and 5c prepared by adding 25 parts by weight of aluminum oxide (Al₂O₃) to 100 parts by weight of aluminum nitride, samples Nos. 15a, 15b and 15c prepared by adding 5 parts by weight of silicon dioxide (SiO₂) to 100 parts by weight of aluminum nitride and samples Nos. 25a, 25b and 25c prepared by adding 25 parts by weight of zirconium oxide (ZrO₂) to 100 parts by weight of aluminum nitride while setting distances A from starting points of circuits of heating elements 2 to ends of substrates 1 a closer to electrodes 3 to 5 mm, 10 mm and 20 mm respectively. Each substrate was assembled into the soldering iron 10 shown in FIG. 3, and the characteristics of the substrate serving as a ceramic heater were evaluated through a procedure similar to that in Example 1. Table 7 also shows the results. TABLE 7 Distance A Power Thermal to End of Temperature Consumption Sample Conductivity Substrate of Electrode at 300° C. No. (W/m · K) (mm) A/B Part (° C.) (W)  2a ⋆99 ⋆5 10 272 113  2b ⋆99 10 20 241 105  2c ⋆99 20 40 182 97  5a 50 ⋆5 10 290 104  5b 50 10 20 281 73  5c 50 20 40 262 52 15a 63 ⋆5 10 280 101 15b 63 10 20 279 70 15c 63 20 40 258 49 25a 65 ⋆5 10 290 102 25b 65 10 20 280 71 25c 65 20 40 270 50

[0049] When gradually increasing the distance A from the starting point of the circuit of the heating element to the end of the substrate closer to the electrode while keeping the length of the substrate constant, the circuit of the heating element is shortened and hence power consumption is reduced as a matter of course. Referring to Table 7, power consumption is excessive in the samples 2a, 2b and 2c having thermal conductivity exceeding the upper limit of the range recommended in the present invention although the temperature of the electrode part does not reach a temperature region facilitating oxidation of the part of the electrode. Similarly, power consumption is excessive in the samples 5a, 15a and 25a not satisfying the relation A/B≧20 between the distance A to the end of the substrate and the thickness B of the substrate. As to the remaining samples, the temperature gradient from the heating element to the part of the electrode is loose and power consumption is suppressed.

[0050] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. A ceramic heater comprising: a ceramic substrate having a certain thickness; a heating element having a circuit formed on the surface of said ceramic substrate; and an electrode formed on the surface of said ceramic substrate and connected to said circuit of said heating element, wherein A and B satisfy a relational expression A/B≧20 assuming that A represents the distance from a contact between said circuit of said heating element and said electrode to an edge of said ceramic substrate closer to said electrode and B represents the thickness of said ceramic substrate, and the thermal conductivity of said ceramic substrate is at least 30 W/m·K and not more than 80 W/m·K.
 2. The ceramic heater according to claim 1 , wherein the material forming said ceramic substrate contains a main component of at least one material selected from a group consisting of aluminum nitride, silicon nitride and silicon carbide and a subsidiary component having thermal conductivity of not more than 50 W/m·K.
 3. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least 5 parts by weight and not more than 100 parts by weight of aluminum oxide added as said subsidiary component.
 4. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least either silicon or a silicon compound of at least 1 part by weight and not more than 20 parts by weight in terms of silicon dioxide added as said subsidiary component.
 5. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least either zirconium or a zirconium compound of at least 5 parts by weight and not more than 100 parts by weight in terms of zirconium oxide added as said subsidiary component.
 6. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least 15 parts by weight and not more than 30 parts by weight of titanium oxide added as said subsidiary component.
 7. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least 5 parts by weight and not more than 20 parts by weight of vanadium oxide added as said subsidiary component.
 8. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least 5 parts by weight and not more than 10 parts by weight of manganese dioxide added as said subsidiary component.
 9. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least 5 parts by weight and not more than 15 parts by weight of magnesium oxide added as said subsidiary component.
 10. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of aluminum nitride as said main component and at least 1 part by weight and not more than 10 parts by weight of at least either an alkaline earth element or a rare earth element of the periodic table added as a sintering agent.
 11. The ceramic heater according to claim 10 , wherein said alkaline earth element is calcium.
 12. The ceramic heater according to claim 10 , wherein said rare earth element is neodymium or ytterbium.
 13. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon nitride as said main component and at least 2 parts by weight and not more than 20 parts by weight of aluminum oxide added as said subsidiary component.
 14. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon nitride as said main component and at least 5 parts by weight and not more than 20 parts by weight of zirconium oxide added as said subsidiary component.
 15. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon nitride as said main component and at least 10 parts by weight and not more than 30 parts by weight of titanium oxide added as said subsidiary component.
 16. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon nitride as said main component and at least 5 parts by weight and not more than 20 parts by weight of vanadium oxide added as said subsidiary component.
 17. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon nitride as said main component and at least 5 parts by weight and not more than 10 parts by weight of manganese dioxide added as said subsidiary component.
 18. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon nitride as said main component and at least 10 parts by weight and not more than 20 parts by weight of magnesium oxide added as said subsidiary component.
 19. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon carbide as said main component and at least 10 parts by weight and not more than 40 parts by weight of aluminum oxide added as said subsidiary component.
 20. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon carbide as said main component and at least 5 parts by weight and not more than 20 parts by weight of zirconium oxide added as said subsidiary component.
 21. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon carbide as said main component and at least 15 parts by weight and not more than 30 parts by weight of titanium oxide added as said subsidiary component.
 22. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon carbide as said main component and at least 10 parts by weight and not more than 25 parts by weight of vanadium oxide added as said subsidiary component.
 23. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon carbide as said main component and at least 2 parts by weight and not more than 10 parts by weight of manganese dioxide added as said subsidiary component.
 24. The ceramic heater according to claim 2 , wherein the material forming said ceramic substrate contains 100 parts by weight of silicon carbide as said main component and at least 5 parts by weight and not more than 15 parts by weight of magnesium oxide added as said subsidiary component. 